To provide effective circuit interruption in circuit interrupters, fuses, and the like, it is desirable to utilize an arc-quenching material or composition to quench and suppress arcing during electrical contact separation or fuse operation. Of necessity, the arc-quenching materials should include characteristics and properties sufficient for the particular application so as to be effective in quenching arcs via the rapid evolution of quenching gases. Of course, the evolved quenching gases should also be relatively nonconductive. In addition, it is also important that the arc-quenching materials are capable of being molded or otherwise fabricated into suitable articles and shapes having desirable structural properties, thermal stability, and environmental resistance to thermal cycling.
In many circuit-interrupting devices, it is typical to utilize a trailer/liner configuration, as well known in the art, so that the arc is drawn into an annular space defined between the trailer and the liner, each of which is preferably fabricated from an arc-quenching composition. The action of the gases produced by the trailer and/or liner on the confined arc tends to deionize the arc and force its extinction. Examples of trailer/liner configurations are shown in the following U.S. Pat. Nos. 2,351,826; 2,816,980; 2,816,978; 2,816,985; 4,103,129; and 3,909,570 and in Descriptive Bulletin 811-30 of S&C Electric Company, Chicago, Ill. Similarly, in high-voltage fuses, which also can be characterized as circuit interrupters, a sleeve or liner surrounds the path of the arc during fuse operation with the sleeve or liner being fabricated from an arc-extinguishing material. Reference may be made to U.S. Pat. Nos. 3,629,767 and 4,307,369 for an example of fuses of this type surrounded by arc-extinguishing sleeves or liners.
Typical arc-extinguishing materials and their properties are disclosed in the following U.S. Pat. Nos. 3,582,586; 4,251,699; and 4,444,671. One composition in U.S. Pat. No. 3,582,586 includes melamine and polyethylene. While this composition is generally suitable for various applications and exhibits desirable arc-quenching properties, for many applications, it would be desirable to achieve a composition with improved mechanical characteristics and environmental resistance to thermal cycling while maintaining the desirable arc-quenching characteristics.
One of the most effective arc-interrupting compounds used in this art for arc-quenching is melamine (C3N6H6) which is a white crystalline powder having a melting point of about 350° F. and sublimes at its melting temperatures and below. Other, related nitrogen-containing compounds are also recognized in the prior art as arc-quenching or arc-interrupting compounds and are disclosed in Amundson et al U.S. Pat. No. 2,526,448. Melamine and its related compounds have excellent arc-interrupting characteristics but suffer from extreme structural weakness, so that they cannot be molded or pressed into satisfactory structural shapes except in combination with a suitable binder.
For a binder to be most effective in an arc-quenching or arc-interrupting composition it should volatilize or decompose in the presence of an electric arc, as does melamine. The binder, however, does not necessarily have to provide any arc-interrupting or arc-extinguishing characteristics to the composition, since, in some cases, the arc-interrupting characteristics of the melamine included in the composition is sufficient for arc-interrupting purposes. The binder, therefore, is primarily included for purposes of providing the melamine-containing composition with sufficient moldability and to provide a molded structure of sufficient physical strength, physical and chemical stability, and electrical insulating properties to provide a structurally sound, molded product. The physical strength of the molded product is most evident in its tensile strength, its percent elongation, and the amount of energy required to break the molded structure, or impact strength.
Structural damage, i.e., cracks have been found in prior art devices containing polyethylene as its primary binder material, and such damage is unacceptable in this art, since the break point allows another air space for the gases and arc to fill, thereby significantly lessening the arc-interrupting properties of the arc-interrupting device. Further, failed arc extinguishing compositions that contain melamine usually fail because the pressure wave associated with the arc causes the composition to physically break before it has an opportunity to extinguish the arc. The arc-extinguishing compositions described herein extinguish the arc without physically breaking. Thermoplastic polymeric binders have been found to be the most useful in arc-interrupting compositions based upon melamine or similar compounds, since the thermoplastic binders volatilize or decompose in the presence of an electric arc at lower power conditions than necessary to sublime melamine thereby producing large volumes of gas to drive the melamine into the core of the arc and to extinguish the arc under a wide range of power conditions. Further, the thermoplastic binders provide compositions with good molding characteristics, stability and electrical insulating properties.
Typical thermoplastic polymeric resins known to be useful as binders in melamine-based arc-interrupting compositions include polyethylene, polypropylene, polytetrafluoroethylene, acrylics, polystyrene, cellulosics polyamides (nylons), polyacetals (DELRIN), polyphenylene oxides, blends such as ABS, and polyimides. Other binders, such as thermosetting resins, epoxy resins, polyester resins, phenolic resins, and the like, also are known to be useful as binders in arc-interrupting compositions. It is also known to include elastomeric, rubber-like materials as a portion of the binder in melamine-based arc-interrupting compositions such as butyl compounds, isoprene-based compounds, neoprene-based compounds and other synthetic elastomers.
In this assignee's U.S. Pat. No. 4,975,551, there is disclosed a binder comprising a carboxylic acid group-containing polymer, particularly a copolymer of two different monomers, at least one of which contains a carboxylic acid moiety, such as an ethylene acrylic acid copolymer. As disclosed, the carboxylic acid functionalities of the binder interact with arc-extinguishing compounds having carboxylic acid-active sites, such as amine, thiol, alcohol, halogen, and the like sites, to provide added physical strength and stability to the composition. The molded composition, including the arc-interrupting compound and the binder, maintains excellent arc-interrupting capability, chemical stability and electrical insulating properties as well as increased physical strength.